1. Field of the Invention
The present invention relates to a method of controlling electron beams in an image display apparatus for use in a video equipment.
2. Description of the Related Art
Conventionally, a cathode-ray tube is mainly used as the image display element of a color television, however, the cathode-ray tube is much greater than the screen in depth. Therefore, it is impossible to manufacture a thin television receiver. In order to manufacture a thin television receiver, display elements such as an EL display element, a plasma display element, and a liquid crystal display element have been developed. But none of them are preferable in luminance, contrast, and a color reproducibility.
In order to solve the above-described problem, the following image display apparatus of a color television has been developed. The image plane thereof is divided into a plurality of sections of a matrix configuration so that a thin and flat device displays an image utilizing electron beams which form an image equal to that of the cathode-ray tube in quality. Each section is scanned by deflected electron beams so that a fluorescent substance emits a light.
The above-described conventional image display apparatus is described in detail with reference to the drawings.
Referring to FIG. 2 showing the inner construction of the image display apparatus, it comprises a backplate 1; a line cathode 2 serving as an electron beam source; a leading electrode 3; a signal electrode 4; a focusing electrode 5; horizontal deflecting electrodes 6a and 6b; vertical deflecting electrodes 7a and 7b; and vacuum glass containers 8a and 8b accommodating the above components.
Describing the outline of this apparatus, a plurality of rows of electron beams generated by the line cathode 2 are horizontally sequentially introduced through the through-holes of the leading electrode 3. The electron beams are sequentially controlled by the signal electrode 4 according to a video signal. Then, the electron beams are electrostatically focused by the focusing electrode 5, and then, electrostatically deflected horizontally by the horizontal deflecting electrodes 6a and 6b and vertically by the vertical deflecting electrodes 7a and 7b. A section 11 of a screen 10 is scanned by one electron beam 9. All sections 11 are connected to each other without any gap therebetween to form an entire image on the screen 10. These electrodes consist of a thin conductive plate and can be laminated with each other.
Therefore, compared with a cathode-ray tube, a thin image display apparatus can be manufactured.
However, in the above conventional image display apparatus, the diameters of electron beams composing a small image plane of a matrix configuration are different from each other in the vertical sections thereof. Thus, the difference in spot diameters does not ensure the uniformity of the image formed on the screen 10. This phenomenon is described with reference to FIG. 3 which is a vertical sectional view showing one electron beam generated by one of the line cathodes 2 of the conventional image display apparatus shown in FIG. 2. The electron beam is electrostatically deflected by the potential difference between the vertical deflecting electrodes 7a and 7b to form trajectories 20 and 21, thus reaching the screen 10. As a result, beam spots 22 and 23 are formed on the screen 10. But when the electron beam is electrostatically deflected, deflection errors (coma) occur because the diameter (d) thereof is great when it passes through the vertical deflecting electrodes 7a and 7b. As a result, compared with the beam spot 22 formed when a deflection amount is small in the vertical direction, the beam spot 23 formed when a deflection error (coma) is great in the vertical direction is not focused. Therefore, the beam spot 23 is vertically elongated or trailed upward. Spots in different configurations appear in succession horizontally on the screen 10. Therefore, when an entire image is viewed, long and narrow non-uniformities having different luminances are observed, thus damaging the uniformity of the image.